The invention relates to inertial devices for measuring rotation by means of a resonant optical cavity in the form of a ring and more particularly concerns active ring gyrometers in which there is an amplifier medium, and more particularly a phase conjugation non-linear amplifier medium which may be of a gaseous or a semiconductor material. The invention relates especially to the manner of ensuring decoupling of the frequencies of two counter-rotative waves so as to prevent them from "locking" under certain conditions.
As is known, optical gyroscopes having a resonant cavity employ the Sagnac effect. This effect, which is caused by differences in relative inertia, has for a result that, when two waves circulate in the same geometrical path in opposite directions, the times they take for traveling through this same geometrical path are different due to the rotation of the system which carries this geometrical path relative to a Galilean reference system. Such differences in traveling time destroy the symmetry of the optical paths "seen", respectively, by each of the two waves.
The article published under the title "Optical Gyroscopes" by J. J. Roland et al. in the review "Optics and Laser Technology" of October 1981, starting on page 239, briefly explains the manner in which optical gyroscopes operate and the different varieties thereof which have been developed.
All the optical gyroscopes operate in accordance with the same principle.
If two beams of coherent light are made to propagate in two opposite directions in the same geometrical path which is closed in the form of a ring, a difference occurs between the beams when the optical paths rotate. A time difference occurs which is proportional to the speed of rotation of the system relative to a Galilean reference system. This time difference of the travel results in a difference in the optical path including a difference of frequency between the two waves propogated in opposite directions. This difference of frequency becomes apparent when the two beams are made to interfere so as to obtain beats representative of the interference.
The mathematical formulation may be expressed as follows.
The difference .DELTA.t, of the travel times of the two waves in opposite directions along the same geometrical path closed in the form of a ring which results in a lack of symmetry in their optical paths, is written: EQU .DELTA.t=4 S..THETA./c.sup.2
in which S is the inscribed area of the ring path, .THETA. is the speed of rotation impressed on the path, and c is the velocity of light.
From this equation, the following operating difference is obtained: EQU .DELTA.p=c.DELTA.t=4 S..THETA./c=4 S .THETA./.lambda.f
in which .lambda. is the wavelength of the beam and f is the frequency of resonance when the system is immobile.
Likewise, the relative variation of the operating difference is expressed as follows: EQU .DELTA.p/p=.DELTA.f/f
and the frequency slip .DELTA.f=4 S..THETA./.lambda.p.
By integrating the above equation with respect to time, a resultant number of fringes N is obtained which is a function of the speed of rotation and written: ##EQU1##
Thus it can be seen that the number of fringes N is proportional to the angle .THETA. through which the geometrical path, or the system carrying the geometrical path, has rotated.
The active resonant ring cavity gyroscopes have shown their worth but are not without drawbacks.
As is known, when the frequency of the beats of the two interfering waves is lower than a certain value, a mutual coupling of the two waves results in their being coupled to each other such that they oscillate at the same frequency. This results in a "locking" wherein the frequency difference, which was proportional to the rotation, disappears. This "locking" phenomenon occurs at low rates of rotation and results in the gyroscope no longer revealing the rotations and, in a sense, becoming "blind" and therefore useless.
This mutual coupling between the two counter-rotative waves is in particular due to the retro-diffusion introduced by the optical components such as the mirrors which define the geometrical closed ring path.
The multiplying coefficient K is, in fact, a function of the rotation of the system and instead of the frequency slip response curve as a function of the rotation being rectilinear throughout, it is hyperbolic at low speeds of rotation and rectilinear at high speeds. It will therefore be understood that between the two limit values, one for each direction of rotation, which mark the boundaries of the "blind" zone, the gyroscope is incapable of revealing a rotation of the system by which it is carried.
One of the most currently employed solutions for overcoming this drawback, consists in subjecting the gyroscope to periodical oscillations alternating at a frequency on the order of a few hundreds Hertz. With this remedy, it is possible to obtain a response which is practically linear for a wide range of operation.
Such an oscillatory swinging, or "periodical bias" is mostly obtained mechanically, for example by means of torsion bars which are made to operate at their frequency of resonance, or "magnetic mirrors" employing the Kerr magneto-optical effect.
As is known, gyrometers having an active laser in the form of a ring employ an amplifier medium which is placed in the ring and whose use involves an electrical discharge in a gas. Consequently, an exterior electrical field must be created to excite the amplifier medium.
The use of a gas as an amplifier medium, most often a He - Ne mixture excited by an electrical discharge, produces other defects of non-reciprocity depending on the direction of travel due, for example, to an imperfect alignment of the resonant cavity, to the flow of the gas subjected to the discharge, or to a thermal sensitivity of the alignment and the flow. Furthermore, the use of such a medium requires the forming of a vacuum which is always delicate, complex and costly.